Everything can drag but because we're an animate object with muscles and bones we have control over ourselves and that affects the natural overlap of the human body

Overlaps are best added towards the end when you have your main objects animated and tied down. Your primary action must be approved/completed! You should only do it once your timing and spacing is complete and finalized.

Overlap can be described as secondary animation.

S curves normally occur in hair/rope/chain and similar moving objects when it changes direction

The most extreme frames of overlap will be a drag

The end of the tail should create an arc throughout the animation

Point the tip back to where it came from on the previous pose

When applying overlap to a tail, hair or spine keep in mind that the base of the tail/hair/spine will move the least and the end will move the most. Kind of like a whip.

And here's a bit of a conversation I had with a fellow iAnimate student (Martyn Smith) who came out with this piece of gold:

I remember asking Stephen (Melagrano):
"What's more important, realism or strong graphic poses?"
he said,
"Strong graphic poses, definitely. Something that reads instantly and clearly."
But then Mike said they'd often throw away graphic poses because it just didn't fit into the flow of things so I guess there's a balance.

Which goes to show that the "guidelines" can be broken. I had followed the "tip should point where it last came from" advice in a recent animation but it just didn't look as good as it could have. I ended up pushing the poses so that the hair and body made stronger S shapes and it worked so much better but the rest of my work on the overlap did follow that piece of original advice.